This invention relates to a novel method for the electrolysis of an aqueous alkali metal chloride solution by use of a cation-exchange membrane. More particularly, the invention is directed to a method for the manufacture of an alkali hydroxide by the electrolysis of a corresponding alkali metal chloride solution in electrolytic cells of the type having anode compartments and cathode compartments separated by a cation-exchange membrane, which method is characterized by having the cathodes positioned and maintained in intimate contact with one surface of the cation-exchange membranes and the anodes in as close proximity as possible to the other surface of the membranes.
The process for the electrolysis of alkali metal chlorides by use of cation-exchange membranes has overcome conventional technical drawbacks and has attained growth as a novel energy-saving approach. Among the salient advantages of this process are the preclusion of the possibility of environmental pollution owing to the disuse of mercury and asbestos, the production of caustic soda of high purity owing to the ability of the cation-exchange membranes to prevent diffusive passage of NaCl from the anode compartments to the cathode compartments, and the liberation of chlorine gas and hydrogen gas both of high purity owing to the perfect separation of the anode compartments and cathode compartments by the intervening cation-exchange membranes. This process is said to have already surpassed the mercury process and the diaphragm process in terms of the total energy cost embracing both steam and electricity.
Need is nevertheless felt for the development of techniques which permit further reduction in the cost of electricity because the proportion of the cost of electricity in the relative production cost is so high to approach the order of 40% in Japan.
Approaches offered for reducing the distances between the anodes and cathodes and consequently reducing the volumes of gases ocurring around the electrodes are effective in lowering voltages involved. Such methods are disclosed in Japanese published unexamined patent application Nos. 80974/1975 and 109899/1975, for example. With these methods, however, although the distance between the anodes and ion-exchange membranes are indeed small, the distances between the cathodes and ion-exchange membranes are still large and the reduction in voltages is not sufficient.
With a view to shortening the distances between the cathodes and ion-exchange membranes, Japanese published unexamined patent application No. 17375/1979 suggests an idea of filling a lot of granular electric conductors between the cathode and the membrane and causing the granular substances to function as cathodes. This method, however, suffers from a disadvantage that heavy contact resistance occurs between the adjoining individual granules and between the granules and the cathode.
Japanese published unexamined patent application No. 47877/1979 teaches a method of mechanically pressing the anodes and cathodes into intimate contact with the ion-exchange membranes with the force such as of springs. Although this method is free from the disadvantage of heavy contact resistance suffered by the method of Japanese published unexamined patent application No. 17375/1979, it is not allowed to obtain an appreciable reduction in the inter-electrode distances owing to the limits to the fabricative precision of electrodes. Inevitably, therefore, the reduced distances average 1 mm and, at times, increase to the order of 2 mm.
Electrolytic systems using still smaller interelectrode distances have been suggested by Japanese published unexamined patent application Nos. 78788/1977 and 52297/1978. In these systems, anodes are embedded in one surface of the membranes and cathodes in the other surface of the membranes. According to these methods, since the inter-electrode distances equal the thicknesses of the ion-exchange membranes, the systems applied to the electrolysis of alkali metal chlorides are expected to afford reduction in voltages involved. They nevertheless entail the disadvantages: (1) Although the electrolytic voltage is low where the current density falls in the range of low levels, it tends to increase as the current density rises. (2) The current efficiency also tends to decrease as the current density rises. (3) The oxygen gas content in the chlorine gas is greater than in the operation by the conventional method. With the conventional method, the oxygen content is generally below 1%, whereas with the methods under discussion, it is as high as several %. Particularly when the current density is increased, this value aburptly increases possibly to surpass the level of 10%. This increase in the oxygen content deprives the ion-exchange electrolytic process of one of its characteristic features. (4) Use of expensive devices as the current collectors on the anode side is inevitable and the resistance loss in the current collectors is large. (5) The systems are difficult to apply to commercial-scale multi-electrode electrolytic cells. When they are incorporated in such multi-electrode electrolytic cells possessed of explosion-bonded partition walls as disclosed by Japanese published unexamined patent application No. 43377/1976 with a view to lowering the resistance loss within the current collectors, the gasket must be given a properly adjusted thickness so that the current collectors come into contact with the anodes and cathodes embedded in the opposite surfaces of the cation-exchange membranes and not inflict a wound upon the membranes. From the practical point of view, therefore, such adjustment proves extremely difficult.